A lead-acid battery has lead positive plates, lead oxide negative plates, and an electrolyte which is a solution of sulfuric acid and distilled water. The plates and the plate separator material between them are arranged in plate packs in plate compartments. During charging, and especially if there is overcharging, some of the water content of the electrolyte will be hydrolyzed into its component hydrogen and oxygen gases. Batteries of the valve regulated maintenance-free type are constructed so that the hydrolyzed gases are recombined under pressure internally within the battery case to reform water, so that normally no water is lost. Since no water is lost, the batteries are "maintenance-free."
Nevertheless, under certain conditions electrolyte can escape, even from a valve regulated maintenance-free battery. While the battery is being charged, and particularly if it is being overcharged, internal gas generation may take place at so high a rate as to exceed the rate at which the battery can internally recombine the hydrogen and oxygen to reform water. Under such conditions the internal gas pressure rises and must be released from the case. Pressure relief valves are built into the battery, often one for each plate compartment, to regulate internal pressure in that compartment and prevent it from becoming excessive. The pressure relief valves are normally closed and are set to open to release pressure in excess of a predetermined maximum. When venting occurs, gas released through the valve can carry with it entrained droplets of the sulfuric acid electrolyte, so that electrolyte is released along with the gas. The escape of sulfuric acid electrolyte from the battery is highly undesirable: it can quickly corrode electrical connections to the battery or attack adjacent electrical circuitry.
The problem of electrolyte leakage through a pressure regulating valve is most likely to arise where the battery is used in an application in which it is not oriented in an upright attitude, as when the battery is on its side or upside down. This arises, for example, where the battery is used in a portable light or as standby power for exit or emergency lighting. An emergency light is often mounted on the wall or ceiling of a room, in such position that the battery is tilted 90% from upright or is even inverted. Normally, emergency lighting batteries are in a float or standby condition in which they are given only a small or trickle charge sufficient to maintain them at full charge without gas venting. However, if there is a failure of external power, the battery may have to provide power for a prolonged period and may become deeply discharged. If a deeply discharged battery is recharged at high current, internal gas pressure from dissociated water molecules tends to rise rapidly and the pressure relief valves may open. If the battery is on its side and particularly if it is in an inverted position, the valves will most likely be covered by liquid electrolyte. Release of gas through the valve can sweep electrolyte out with it, and thereby lead to the problems just noted.
It is known to filter gas and electrolyte released from a valve regulated, non-maintenance free battery in order to remove the electrolyte, as shown in British patent No. 791,139 and Baum U.S. Pat. No. 2,331,450. The filters carry electrolyte-neutralizing compounds such as sodium carbonate that react with the electrolyte to form a harmless salt, while permitting the gas to escape through a vent. However, the salt produced by neutralization tends to clog the filter and eventually to impede gas release, and thus requires periodic cleaning. In a valve regulated battery, it is desirable to provide a means for separating electrolyte droplets from vented gas without at the same time creating salts or otherwise restricting the release of the gas, and without significantly or undesirably increasing the internal pressure in the battery.